"We're headed out to Lake Callabonna, we're going to set up the camp there so that we can start searching for the remains of giant animals," said palaeontologist Dr Trevor Worthy, as he prepared to depart on a recent research trip.

This dry salt lake in north eastern South Australia is regarded as one of the most famous megafaunal sites in the country. It harbours an expansive but undated collection of fossils.

The lake has long offered researchers a wealth of insight into Australia's historic native fauna. Dr Worthy and his team from Flinders University are the latest to undertake an expedition to uncover the many unknowns still harboured in Lake Callabonna.

It is most well-known for the presence of diprotodons, the biggest marsupial that ever lived on earth, which resembled a wombat but of rhinoceros proportions.

But on this particular trip, researchers were focussing on the remains of a lesser known species: an extinct giant bird called a mihirung, which weighed up to 250 kilograms.

"They might have stood the same height as an ostrich but they were probably three or four times the mass," said Dr Worthy.

"Its bones are just massive. Instead of having a long slender drumstick, everything about it is bigger and fatter. It's just a bulky bird and not really a running bird."

Uncovering mihirung mysteries

The remains of this giant bird were first discovered at this lake in 1893 during an expedition by the South Australian Museum.

"They were in the field for 11 months and in that time they got parts of 14 birds, but most of those are fragments, they only came back with, like, the end of a drumstick, for example," said Dr Worthy.

This latest research team aimed to discover a wider collection of mihirung fossils to better grasp the age and structure of the species.

"This mihirung bird is the last of this lineage. They existed in Australia for at least 45 million years and while it's the last one, we still know nothing about much of the skeletons.

"To try and ensure we can put it in the family tree of these birds, we need to get a better grasp on its skeletal morphology."

Finding the skeletons on a dried-up lake 50 kilometres long and 10-15 kilometres wide is no easy task and the team will be looking closely for gizzard stones to identify birds' remains.

"The birds are going to be exposed by the presence of the gizzard stones, which are lots of little stones they hold in the first chamber of their stomach that grind up their food.

"That's a very good clue as to where they are because they don't blow away in the wind and if they're starting to erode out the bed of the lake then there's a bird there."

Once identified, birds will be excavated using small tools, including trowels and dental picks, to carefully expose the bones.

"Once it's exposed it will probably be quite soft and we'll have to harden it on site and wrap it in plaster jackets to bring home."

Finding out fossil ages

It is the lake's soft clay-type sediments which have enabled the unique collection of fossils to become preserved in Lake Callabonna.

Mr Worthy said previous scientific "reconnaissance missions" have revealed dozens of remains from a variety of species.

"Most of these animals appear to have walked out onto the lake for whatever reason, whether they were trying to get between two low ranges of dunes or something, and they eventually got mired in the clay.

"We see the skeletons quite often just stuck there with the four feet poking straight up and all the rest gone. So very often the animal just walked along and got bogged."

From this trip the researchers hoped to get closer to dating this lake site and identifying which period the animals it harbours were dying.

"It's the most important megafaunal site in Australia and we don't know which age it actually is.

"The main tool we have for dating things in the 0-45,000 year period is radiocarbon dating and it seems most of them went extinct a little earlier than this [tool] can capture."

As part of this recent project, new sediment testing technologies will be used to try and offer some more precise dating and resolve the uncertainly behind these animals' demise.

"Capturing the exact age is a challenge and has generated a debate that's been going for a couple of decades now," Dr Worthy said.

"On one hand, we've got people who say probably human arrival and the hunting pressure they imparted resulted in the animal's demise.

"On the other hand, we have researchers who are not so happy with the conclusion of humans killing off the animals and favour other things like climate change."